The essential principles of product design

Most product design advice is really advice about taste. Useful, but hard to hand to someone else. The principles that transfer are duller and more mechanical, and they are usually about what you decide not to build.

The first is that a screen should have one job. Not one element, one job. A screen that answers two questions equally well answers neither quickly, and speed of comprehension is almost always the metric that moves everything downstream of it.

The second is that states are the design. The empty state, the loading state, the error, the one-item case and the two-hundred-item case are not edge cases to be handled later; they are the majority of the time a real person spends in a real product.

The third is that consistency beats cleverness at scale. A slightly worse pattern used everywhere will outperform a better pattern used in three places, because the cost people pay is in relearning, not in the pattern itself.

None of this is a substitute for judgement about the specific thing you are making. It is a way of spending that judgement where it counts rather than on the parts that have already been solved.